The present invention relates to steels for rolling members that are suitable for use as bearings in high-speed rotation at high temperatures as in aircraft jet engines and gas turbines. The present invention also relates to rolling bearings that provide an outer race, an inner race and rolling elements, particularly relating to those rolling bearings which are extended in their service life.
From energy and environmental viewpoints, efforts are recently being made to increase the operating efficiency of aircraft jet engines, gas turbines and other rotating machines and, as a result, the conditions under which bearings are used have become more rigorous than ever, namely, the rotational speed has increased and the field of rotation has become hotter. Conventionally, semi-high-speed steel AISIM50 has been used as a bearing material for high-speed rotation at high temperatures. This steel is intended for use at 200.degree. C. and as long as it is used at this temperature, it is capable of effectively insuring H.sub.R C (Rockwell Hardness in scale C) 60 which is the necessary hardness for bearings; however, it fails to assure the necessary hardness at higher temperatures. Bearings in aircraft jet engines, gas turbines and other rotating machines are predicted for use at temperatures of 300.degree. to 400.degree. C. in the near future; however, in this temperature range, AISIM50 is only capable of assuring hardness in the range form H.sub.R C 55 to H.sub.R C 59 and its life would come to an end very soon. It should also be noted that in the field of high-speed rotation at high temperatures, the precision of bearings per se must be improved and the slightest error leads to a serious accident. An accident to jet engines is fatal to human life whereas an accident to power generation turbines can be a serious social problem; in either case, greater care must be exercised to secure safety than when operating other rotating machines. Therefore, the slightest dents, cracks or other surface flaws in bearings should not directly lead to the entire failure of the bearings. In other words, the core of rolling component parts such as the inner race, outer race and rolling elements of a rolling bearing have desirably not only high toughness but also a sufficient compressive stress in the neighborhood of their surface to withstand the high hoop stress that develops upon high-speed rotation. However, AISIM50 has low toughness since macro carbides are interspersed in the basis structure; in addition, a compressive stress region which develops upon final surface working such as grinding and polishing remains in the surface of AISIM50 but it occurs in a very shallow area.
Under the circumstances, efforts are being made to produce a powder sintered version of AISIM50 by a process that provides preparing a compact of powder feed and performing HIP (hot isostatic pressing) to increase the density of the sinter. The powder sintered steel thus produced is improved in the toughness of the core but the problems of insufficient hot hardness and very shallow surface compressive stress region remain practically unsolved. According to Unexamined Japanese Patent Publication (kokai) Hei 1-201422, it is proposed that in order to refine the carbides in AISIM50, hot homogenization should be performed by first working the steel to a forging ratio of at least 2 and then soaking it at 1150.degree. to 1220.degree. C. for a period of at least 2 hours. However, the problems of insufficient hot hardness and very shallow surface compressive stress region also remain unsolved. Unexamined Japanese Patent Publication (kokai) Sho 63-62847 proposes extending the life of AISIM50 under clean environments by restricting the content of inclusions; however, the hot hardness of the steel is not improved and little is expected in the extension of its life under hot environments by this approach. In addition, the problems of low core toughness and very shallow surface compressive stress region remain unsolved. In recent years, M50NiL which is reduced in the carbon content compared to AISIM50 and which has Ni added to achieve a structural balance has been proposed and is finding increased use; with this new steel, the problems of low core toughness and shallow surface compressive stress region are solved but the problem of insufficient hot hardness is not.
Thus, no steels have yet been developed that have high core toughness, adequately deep surface residual stress region and higher hot hardness and which are capable of withstanding the operation of rotating machines under rigorous conditions that are to be encountered in the future as the rotational speed increases at higher temperatures.
Accordingly, the conventional steels have the following problems. The steel AISIM50 which has conventionally been used as a bearing material for high-speed rotation at high temperatures has poor toughness since macro carbides are present in the basis structure and, considering the future use of bearings at higher temperatures, the hot hardness of AISIM50 is insufficient. Further, it does not have a sufficient residual compressive stress in the neighborhood of the surface to withstand the hoop stress that develops during high-speed rotation at high temperatures.